Perché non è un investimento sicuro.
Una valuta che non è garantita da niente, né da un'istituzione, né da una riserva aurea, non ha di fatto un valore proprio, e infatti basta guardare la chart sull'andamento del valore negli ultimi 11 mesi per capire che come è schizzato può anche precipitare.
Non so quanto ci metta a produrre un bitcoin, ovviamente dipende da CPU, GPU, ecc.
Per dire
bitcoin.stackexchange.com/questions/13161/how-long-does-it-usually-take-to-mine-a-bitco...
For solo mining with a single GTX 680, which will produce 120 Mhashes/s, you would expect to find one block approximately every 98 years.
At the current difficulty, solo mining is inadvisable under practically all circumstances. Pooled mining should be done instead.
Furthermore, mining utilizing graphics cards is rapidly losing viability with the release of FPGA and ASIC-based mining hardware, which operates at far greater efficiency. Unless you have exceptionally cheap electricity or have a use for the heat output from the graphics card(s) (e.g. heating your house for winter), graphics card mining is unlikely to be profitable for you.
Additionally, mining with NVIDIA graphics cards has pretty much never been worthwhile, as they are far slower and less efficient than comparable ATI/AMD hardware, conflating the above issues with graphics card mining.
Interestingly enough, using a pool wouldn't actually make a difference -- it would just allow you to get your rewards spread out over the 98 years instead of all at once. So, let's say you do mine an entire block that's now worth 25 bitcoins, over 98 years. That works out to be .000000485032112 bitcoins per minute, or approximately .00000485032112 bitcoins per block. In short, as @Murch points out, it's probably not worth it to mine using that hardware. Sorry. :/
So if you make 0.000000485032112 bitcoin per minute, and let's say 1 bitcoin is $1000. So your making $0.000485032112 per min, or $0.02910192672 per hour or ~$0.69 per day, but the power costs of running the PC don't make it worthwhile.
Go to
bitminter.com/test and click the "test start" button. If you have Java installed the miner should launch. Click "engine start" on your GPU(s) to start mining and the GUI will show how many bitcoins per day you will make (on average).
Note that you are actively mining in a pool without getting paid. This test page launches a version of the miner which is only meant to be used for a short time for testing.
bitcoin.stackexchange.com/questions/118/how-much-bitcoin-will-i-mine-right-now-with-ha...
teksyndicate.com/forum/general-discussion/how-long-get-one-bitcoi...
Average gamer pc, according to some tests what i read somewhere (so it is propably inaccurate) is 0.01 Bitcoin a day.
AMD cards are usually better for bitcoin mining, and it is not worth it, since your power consumption goes way up, and you need something with 3+ GPUs to make some noticable profit. (unless the bitcoin price rises again)
Theres a website with some info on it,
bitclockers.com/calc
Well right nowm with hash rate around 120, you would get 0.007-0.008 bitcoin a day (24h)